Sensing is ubiquitous in modern life, with applications ranging from household and medicine sensing to sensing used in navigation and manufacturing. Memory has become a central part of information storage, communication and processing. Classical sensors and memory have made important contributions to modern life, but as technologies continue to become smaller, faster and more intelligent, the demands for highly sensitive sensors with higher spatial resolution and for high density memory with long lifetimes are increasing.
Quantum information technology is based on the use and manipulation of quantum states, and they offer exceptional increases in device capability and performance. Quantum sensors can provide great increases in sensitivity and spatial resolution, and quantum memory can make possible ultra-dense, long lived memory. Physical implementations for quantum information technology include those based on trapped atoms and ions, superconducting loops, quantum dots and defect states in solid state materials.
Sensors based on atoms and ions have limited spatial resolution and scalability, and superconducting loops and quantum dots require extensive low temperature cooling apparatus and low spatial resolution. Defect states in solids have come to be of interest for sensing and for memory because of the opportunity for room temperature operation, scalability, and high spatial resolution. The nitrogen-vacancy defect state in diamond has attracted considerable interest recently for sensing and memory. However, diamond remains expensive, challenging to fabricate and integrate into current technology, and in its natural state has information losses due to interactions with nuclei.
Technologies based on quantum information are recently opening a range of new opportunities from secure communications to quantum computing. Quantum sensing using entangled entities such as spins, atomic excitations, and photons can provide vastly improved sensitivities compared to classical technologies. Sensing using defect spin states in semiconductors is particularly important in part because of its potential for high spatial resolution and for integration with existing solid state technologies. See J. R. Weber et al., “Quantum computing with defects,” Proc. Natl. Acad. Sci. 107, 8513 (2010); E. Togan et al., “Laser cooling and real-time measurement of the nuclear spin environment of a solid-state qubit,” Nature 478, 497 (2011); H. Bernien et al., “Heralded entanglement between solid-state qubits separated by 3 meters,” Nature 497, 86 (2013); M. S. Grinolds et al., “Nanoscale magnetic imaging of a single electron spin under ambient conditions,” Nature Physics 9, 215 (2013); G. Balasubramanian, “Nanoscale imaging magnetometry with diamond spins under ambient conditions,” Nature 455, 648 (2008); and F. Shi et al., “Single-protein spin resonance spectroscopy under ambient conditions,” Science 347, 1135 (2015).
Room temperature magnetic and strain sensing are being currently investigated using spin-1 and inter-valley spin states, e.g. nitrogen-vacancy (NV) deep color centers in diamond, see F. Dolde et al., “Electric-field sensing using single diamond spins,” Nature Physics 7, 459 (2011), and phosphorous shallow donors in silicon, see C. C. Lo et al., “Hybrid optical-electrical detection of donor electron spins with bound excitons in silicon,” Nat. Mater. 14, 490 (2015) and Ö. O. Soykal, “Sound-Based Analogue of Cavity Quantum Electrodynamics in Silicon,” Phys. Rev. Lett. 107, 235502 (2011) (“Soykal 2011”), that require difficult micro-fabrication processes and experimentally challenging detection techniques.
New concepts and approaches have the potential to move quantum sensing forward to higher sensitivities in systems that are easier to implement.
The technologically important wide band gap silicon carbide (SiC) has mature growth and microfabrication technologies and favorable optical emission wavelengths. See W. Koehl et al., “Room temperature coherent control of defect spin qubits in silicon carbide,” Nature 479, 84 (2011); A. Falk et al., “Polytype control of spin qubits in silicon carbide,” Nat. Commun. 4, 1819 (2013) (“Falk 2013”), D. Christie et al., “Isolated electron spins in silicon carbide with millisecond coherence times,” Nat. Mater. 14, 160 (2014); N. T. Son et al., “Divacancy in 4H-SiC,” Phys. Rev. Lett. 96, 055501 (2006); and A. L. Falk et al., “Optical Polarization of Nuclear Spins in Silicon Carbide,” PRL 114, 247603 (2015) (“Falk 2015”), B. S. Song et al., “Demonstration of two-dimensional photonic crystals based on silicon carbide,” Opt. Express 19, 11084 (2011); R. Maboudian et al., “Advances in silicon carbide science and technology at the micro- and nanoscales,” J. Vac. Sci. Technol. A 31, 50805 (2013), and P. G. Baranov, “Silicon vacancy in SiC as a promising quantum system for single-defect and single-photon spectroscopy,” Phys. Rev. B. 83, 125203 (2011).
Uniaxial silicon carbide (4H-SiC) exhibits a negatively charged silicon monovacancy VSi− having spin-3/2 optical transitions. See E. Janzén et al., “Ihe Silicon Vacancy in SiC,” Physica B 404, 4354 (2009); H. Kraus et al., “Room-temperature quantum microwave emitters based on spin defects in silicon carbide,” Nature Physics 10, 157 (2014) (“Kraus, Nature Physics 201.4); M. Widmann et al., “Coherent control of single spins in silicon carbide at room temperature,” Nat. Mater. 14, 164 (2015); S. G. Carter et al., “Spin coherence and echo modulation of the silicon vacancy in 4H-SiC at room temperature.” Phys. Rev. B 92, 161202(R) (2015); and Ö. O. Soykal, “Silicon vacancy center in 4H-SiC: Electronic structure and spin-photon interfaces,” Phys. Rev. B 93, 081207 (2016) (“Soykal 2016”).
For the VSi− defect, we find an unexpected avoided crossing of its ground-state (GS) spin states forming a naturally entangled Λ-type system leading to a significant increase in sensitivity to magnetic fields. Such an avoided crossing has been observed recently. See D. Simin et al., “All-optical dc nanotesla magnetometry using silicon vacancy fine structure in isotopically purified silicon carbide,” arXiv:1511.04663v1 [cond-mat.mtrl-sci] (2016).